The present invention relates to a fabrication process for a semiconductor device, especially an insulated gate semiconductor device and a semiconductor device fabricated using the fabrication process.
In a semiconductor integrated circuit device, especially a large scale integrated circuit device (LSI), fabricated on one semiconductor substrate, mixedly incorporated are insulated gate transistors with respective different performance requirements such as a combination of devices having respective requirements of a high speed operation and low power consumption, or a combination of devices with respective high and low threshold voltages, as insulated gate semiconductor devices constituting a memory mat section, an I/O circuit section and peripheral circuit sections thereof, for example. In order to realize such a semiconductor integrated circuit device, it is effective to adopt insulated gate semiconductor devices having respective different thickness of a gate insulating film.
For example, in JP-A-S61-194770, description is given of MOSFETs having respective gate oxide films different in thickness and a fabrication method therefor. A detailed disclosure is further given therein that a surface of a semiconductor substrate is oxidized to form a first gate oxide film, a photo-etching treatment using a photoresist film is applied thereon to selectively remove parts of the first gate oxide film and expose the surface of the semiconductor substrate corresponding to the parts, then the remaining photoresist film is removed, and thereafter the surface of the semiconductor substrate is again oxidized to form a second gate oxide film of a thickness different from that of the first gate oxide film.
Further, in JP-A-H8-130250, a fabrication method in which gate oxide films different in thickness are ensured so as not to be put into contact with a photoresist is disclosed in understanding that according to such a prior art method, a gate oxide film is contaminated by impurities in a photoresist during photo-etching, which in turn causes defects to occur. In the publication, a further detailed description is given of the fabrication method: A surface of a semiconductor wafer on which a gate oxide film is to be formed is covered with a silicon nitride film and in this condition, selective oxidation to form a field oxide film for device isolation is performed on the wafer. Thereafter, photo-etching is applied to remove the silicon nitride film from sites on each of which a thicker gate oxide film is to be formed and the thicker gate oxide film is grown in each site by thermal oxidation. After the growth of the thicker gate oxide film, all the silicon nitride film are removed by wet etching from the surface of the wafer. In this condition, by thermal oxidation, a thinner gate oxide film is grown while on the other hand, a thicker gate oxide film is further grown to increase a thickness.
A photoresist has carbon as a main constituting element and the carbon degrades insulating film characteristics when it is attached on or impregnated into the film.
As one of solutions for this problem, a process is conceived in which the attached or impregnated carbon on or in the insulating film is perfectly removed. While etching and cleaning of a silicon oxide film is required for removal of the carbon, requirements arise to immerse a silicon wafer into various kinds of aqueous solutions in a state that a gate oxide film is exposed by removal of a resist after photoetching, in steps such as first SC-1 cleaning, SC-2 cleaning, second SC-1 cleaning, then dilute hydrofluoric acid cleaning and the like, which will be described later. Further, it was found that in the steps, modification in characteristics of the gate oxide film occurs; therefore reliability of the film is reduced. To be concrete, an electric field at which a large area MOS capacitor causes its dielectric breakdown was measured on devices that had received the treatment of aqueous solutions and it was found that more among the devices tested were low in breakdown voltage of a gate oxide film. In addition to this, it was confirmed that especially, when an extremely thin gate oxide film in the range 2 to 6 nm is subjected to etching and cleaning using such aqueous solutions, defects in the gate oxide film are increased in number to affect device characteristics.
Accordingly, new techniques have been required that a gate oxide film is formed without exposing its surface to such aqueous solutions, or alternatively, a gate oxide film is achieved such that deterioration in characteristics thereof is a little even if exposed to such aqueous solutions. For example, while useful is cleaning with an aqueous solution such as of ammonia and hydrogen peroxide water, of sulfuric acid and hydrogen peroxide water or of ozone and sulfuric acid, but in the cases, it was confirmed that modification in characteristics occurs in a gate oxide film to some extent.
On the other hand, as another solution, a technique is conceived in which a photoresist film has no chance to be put into contact with a gate oxide film, but it was found that when in a condition that a thick gate oxide film is exposed, all the silicon nitride film is removed from a surface of the wafer by wet etching as described in the specification of the above described JP-A-H8-130250, especially in the paragraph [0028] thereof, the thick gate oxide film is exposed to an aqueous solution; therefore, defects arise in the gate oxide film as described above, which in turn, affects device characteristics adversely.
A demand has been very recently built up for a large scale semiconductor integrated circuit device (LSI) described above utilizing insulated gate FETs with, as a gate insulating film, a stacked structure including a thin film made of silicon oxide (SiO2) and a thin film made of silicon nitride (Si3N4) aggressively. A further demand has been piled up for constitution of desired circuits or systems using a combination of FETs of this type and FETs with, as a gate insulating film, a thin film made of silicon oxide only in a mixed manner. While as a formation method for the silicon nitride film, a chemical vapor deposition method has been generally used in which dichlorosilane and ammonia are a source gas, this method has been found to be problematic when adopting in mass production of LSIs in that a deposition thickness of a silicon nitride film is largely affected by a condition of an underlying surface, which results in a wide spread of thickness variation of a gate insulating film and therefore, greater dispersion in characteristics such as a threshold voltage among many devices.
It is an object of the present invention to provide a fabrication method for an insulated gate FET in which a silicon nitride film is formed with a small thickness variation; therefore, threshold voltages of insulated gate FETs fall in a narrow range and the above described degradation in characteristics of an device is suppressed to be small.
It is another object of the present invention to provide a fabrication method for a semiconductor integrated circuit device using insulated gate FET""s in which even when a wet treatment is applied in forming gate insulating films having respective different thickness on the same wafer, the gate insulating films maintain high reliability and furthermore a narrow range of thickness variation of devices can be ensured.
An aspect of the present invention is directed to a fabrication method for a semiconductor device including the steps of: forming a first silicon oxide film constituting a part of a gate insulating film on a semiconductor surface; forming a silicon nitride film by means of a chemical vapor deposition method using monosilane and ammonia as a source gas; forming a photoresist film selectively on the silicon nitride film thus formed; removing a part of the silicon nitride film not covered by the photoresist film and the first silicon oxide film beneath the part of the silicon nitride film to expose the semiconductor surface; oxidizing the exposed semiconductor surface after removal of the photoresist film to form a second silicon oxide film with a thickness different from that of the first silicon oxide; and forming gate electrodes on the insulating films.
According to such a fabrication method of the present invention, fabricated are an insulated gate semiconductor device having a gate insulating film comprising a silicon oxide film containing substantially no carbon and a silicon nitride film containing substantially no chlorine deposited on the silicon oxide film; and a semiconductor integrated circuit with the insulated gate semiconductor devices integrated on one substrate.
The foregoing and other objects, advantages, manner of operation and novel features of the present invention will be understood from the following detailed description when read in connection with the accompanying drawings.